Abstract: A large-scale landslide disaster of about 3.1 million m3 happened in Darong village of Siyang town,Cengong country on June 29,2012. Darong landslide is in the accumulation zone of ancient landslide. Based on the analysis of failure characteristics and geological prototype of landslide,the failure mode is qualitatively judged as creep sliding-fracturing-traction sliding. Under the action of unreasonable artificial filled soil in landslide promoter region and heavy rainfall,the base of slope had creep deformation and became unstable,which weakened the supporting force in the base of slope of the main slide area. Consequently,sliding and instability happened in the lower part of the main slide area,which thus induced the upper right side of the main slide to be gradually unstable. Slip zone is located in highly weathered bedrock underlying. The left side of the main body slipped to west,and the right one slipped to southwest. Based on coupled numerical analysis of stress field and seepage field,the instability process and generation mechanism of the landslide are reappeared. The study on formation mechanism of Darong landslide provides an important reference for analogous landslide analysis and identification in western mountains.